This filter was found in an American publication from the 1980s. The circuit can be implemented with any operational amplifier. The power supply must be symmetrical.
Found in a 1965 Radio Shack documentation this oscillator can be used with modern general purpose...
In the figure we have the way to use a single control voltage to act on two varicaps...
Found in a 90s documentation, this switch that is also sensitive enough to operate capacitively uses...